International Society for Soil Mechanics and Geotechnical Engineering
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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 1/18 Liquefaction of Saturated Sandy Soils Liquéfaction de sols sableux saturés by V. A. F lorin , Professor, Doctor of Technical Sciences, Associate Member of the USSR Academy of Sciences, Institute of Mechanics of the USSR Academy of Sciences, and P. L. I vanov , Docent, Candidate of Technical Sciences, Leningrad Polytechnic Institute. Summary Sommaire The authors give results obtained from theoretical and Le présent rapport contient quelques résultats des études experimental research on the liquefaction of saturated sandy théoriques et expérimentales des phénomènes de liquéfaction soils undertaken by them, in the Laboratory of Soil Mechanics de sols sableux saturés qui ont été effectuées aux Laboratoires of the Institute of Mechanics of the U.S.S.R. Academy of Sciences de Mécanique des sols de l’institut de Mécanique de l’Académie and in the Leningrad Polytechnic Institute. des Sciences de l’U.R.S.S. et de l’institut Polytechnique de Laboratory investigations have been carried out on various Leningrad. kinds of disturbances affecting saturated sandy soils and causing Les auteurs décrivent des études de Laboratoire de la liqué their liquefaction. The authors also give the results of studies faction de sols sableux soumis à diverses influences. on the grain-size composition and density of sands, shape of Ils exposent l’influence sur les phénomènes de liquéfaction de particles, entrapped gas content, initial stress condition, intensity la granulométrie et de la densité apparente des sables, de la forme and character of disturbances causing a collapse of the structure des grains, de la teneur en gaz occlus, de l’état initial de contraintes of the soil. Theoretical relationships are given which connect du sol, de l’intensité et du caractère des influences provoquant the process of consolidation in the liquefied soil with the time la rupture de la structure du sol. during which the sand is in a liquid state. These relationships Ils proposent des relations théoriques, relatives au processus largely determine the probable movements of both structures de consolidation des particules d’un sol liquéfié permettant en and liquified masses of soil. particulier, le calcul de la durée pendant laquelle le sable se Finally, the « method of standard blasting » worked out by trouve à l’état de liquéfaction. Ces relations déterminent en V. A. Florin and widely used in the Soviet Union is recommended grande partie les mouvements possibles d’ouvrages et de masses as being suitable for field investigations of the phenomenon de sol liquéfié. reviewed. Pour les études « in situ » du phénomène de liquéfaction, on propose d’employer « la méthode d’explosions uniformes » mise au point par V. A. Florin. La méthode est largement employée en U.R.S.S. The liquefaction of saturated sandy soils may be described different areas in homogeneous soil, regardless of the static as the mechanical breakdown of the structure of the sand homogeneity of the total volume of soil, the stress condition as well as by physical and chemical processes which tend to is different for each individual particle. Different shape and occur in soils, and in particular, by thixotropy. (This is the size of isolated particles account for a different num ber of property possessed by some gels of becoming fluid when contacts between them and different stresses. As a result, even stirred, and of returning to the jelly state when stirring ceases). insignificant disturbances like filtration will cause not only This paper discusses only those cases of soil liquefaction complete but also partial collapse of the structure of the sand which are observed in pure sands and which are of a m echa due to displacements of individual unstable particles of sand nical nature. The phenomenon of complete or partial loss of subjected to minimum stress. By averaging the variable the bearing capacity of a soil, also the transition of the sand quantity of the displaced sand particles, the authors obtain into the state of flow due to collapse of the structure and dis “the degree of break-down”, of the structure of the sand and as placements of grains of saturated sand followed by the form a a result “the degree of liquefaction”, of the sand. T he degree of tion of a new, denser sediment and by the decrease o f the breakdown of the structure, or liquefaction may be expressed porosity of the sand, should be described as liquefaction of as a percentage of the breakdown of contacts, displacem ent sandy soil. of all particles and complete liquefaction of the soil. Liquefaction consists in breaking down the structure, and The effect of liquefaction on the strength and stability o f liquefying the soil mass itself with subsequent consolidation. structures is evaluated not only by its amount of liquefaction, Thus the conditions necessary for liquefaction are : the but also by its character. The possible movements o f structures collapse of the structure, with the possibility of sand consolida are largely influenced by the time during which the sand tion and either partial or complete saturation of the sand in a liquid state and by the viscosity of the liquefied mass of with water. the soil. Because of the great variety of factors causing the collapse The time of liquefaction depends upon the thickness of of a sand structure, the authors propose that the criterion of the liquefied stratum of the sand, the permeability o f the collapse of the soil structure should be not the “critical void sand, variation in the volume of voids in the process of ratio” or the density of the sand, but “critical” values of the consolidation, intensity of draining, location of the drainage intensity of dynamic disturbance, stress condition of the soil system in the body of a structure and the duration of dynamic or weight of surcharge, and hydraulic gradient of w ater flow load causing collapse of the sand structure. through it. For several years the authors carried out numerous labora Sands may pass into a state of either complete or partial tory and large-scale field investigations on several large hydro liquefaction. Considering individual particles of the sand or electric developments. Under laboratory conditions, tests 107 were performed on special impact-and-vibrating tables and The initial distribution of head in a stratum of com pletely in flumes. Tests were also carried out under practical cond liquefied sand may be expressed by the following equation itions on sands which were placed both naturally and hydrau- (Fig. 2) : lically. The main features of sand transformation into the liquefied h = ^?(.S — x) + x , (i) state are the decrease of its porosity and a temporary increase T w of poor water pressure. Under these conditions, measurem ents where : have been made of changes in the porosity of the sand by Yj, = y' + y ,„ — unit weight of completely liquefied soil, using deeply sited reference points and an electrical m ethod Y' — unit weight of suspended sand, to measure porosity, based on the relationship between Y,„ — unit weight of water. the specific electrical conductivity of the soil and that of the pore water. Simultaneously changes in the pore water pres U— 5 — I sures have been measured using special membrane-type inductive cells. In addition, observations were made on the velocity at which heavy bodies sank into the sand. In impulse tests, loose saturated sands were readily concer ted into a completely liquefied state over the entire depth of the stratum. The curves representing maximum excess hydraulic pressure were of triangle shape (Fig. 1 a) and the Fig. 2 Distribution of heads in a stratum of completely liquefied sand. Répartition des charges dans la couche de sol totalement liquéfiée. The density of the sand changes, in general, at the contact 0 5 10 15 20 surfaces between the liquefied and non-liquefied sand stratum. excess poze pzessuie . cm. of w atez caCumn Composing the equation of water equilibrium in elem entary Fig. 1 Fluctuations of maximum excess water pressure due to boundary layer, and considering the conditions for seepage successive impact influences and consolidation of of the squeezed water through a medium, we obtain the follo a sand stratum in completely liquefied state. wing : Variations des surpressions maxima dans l’eau sous Yu l’effet de percussions successives et consolidation de (2) la souche sableuse totalement liquéfiée. K y ' 1 — « where : pressure corresponded to the weight of the waterlogged K — coefficient of seepage, stratum with suspended sand particles (y p <=» 2 gm/cm3). W ith n0 — initial porosity, the increase of sand density, the influence of stress condition n — porosity after sand consolidation. of the sand determined by the weight of overburden tends to be greater, and in the lower layers of the soil no liquefaction The curves representing heads and velocity of movem ent occurs. Because of the pressure of contact between sand p a r of the boundary of the liquefied soil obtained by equation (2) ticles, greater intensity of dynamic disturbance is required agree very well with the experimental values. to break down the structure of the sand than the structure In contrast to shocks, vibrations do not cause liquefaction of its upper layers, and as a result the curves representing over the whole stratum of the soil. The process proceeds in maximum excess pressures at subsequent impulses are of layers : the first vibrations liquefy the upper layer which trapezoidal shape (Fig.